The invention relates to a thermal regulating composition that can be used, for example, as a catalyst system in a fuel processor for a fuel cell system.
Fuel cells are an increasingly popular power generation technology, where chemical reactions are utilized to produce electricity. The reactants are typically hydrogen and oxygen. Along with the electricity generated, the sole reaction-product is water. Hydrogen for such fuel cells may be supplied by chemically converting a fuel such as natural gas, propane, gasoline, diesel, methanol, etc., into a hydrogen-rich stream. This process is typically referred to as fuel processing, and the hydrogen-rich stream is typically referred to as reformate.
The catalyst systems used in fuel processors generally include a dispersion of small catalyst particles on a support material. It is generally desirable to minimize the size of the catalyst particles that are used in order to maximize the surface area of catalyst that is provided to promote a given reaction. However, small metal particles, such as those typically used as catalysts, may tend to be pyrophoric, meaning that they will spontaneously and rapidly oxidize when exposed to oxygen or air. Oxidation is exothermic, meaning that the reaction releases heat energy. Pyrophoricity tends to increase as smaller particles are used, and some metals (e.g., non-precious metals such as iron and copper) may tend to be more pyrophoric than others. In some cases, the heat generated by this oxidation may pose a fire or other safety hazard, or may damage the catalyst configuration itself.
Catalysts subject to such concerns are typically pre-reacted with oxygen in a controlled environment before they are handled. For example, a catalyst may be oxidized slowly in a dilute oxygen atmosphere to avoid overheating, and may then be shipped in a relatively non-reactive oxidized state (referred to as the oxidized state). Since catalysts in an oxidized state generally have diminished catalytic effectiveness or no effectiveness at all, they are typically reduced or activated before they can be used (referred to as the reduced, or active state). This generally involves flowing hydrogen or another reducing agent across the catalyst at an elevated temperature (e.g., over 200° C.), in order to react away the oxidation layer. This reduction (activation) step is also exothermic, and may need to be controlled (e.g., by using diluted hydrogen) to avoid overheating.
One reason why catalyst overheating can be a problem, and thus why catalyst temperature control is important, is because some catalysts will lose their catalytic effectiveness if they are overheated. For example, when copper-based catalyst particles are heated to over 400° C., the particles may tend to sinter (also referred to as densification), meaning that small particles will tend to combine into larger particles. Thus, this temperature may be referred to as the sintering temperature of this material. Such sintering can reduce the surface area of the catalyst, thereby reducing its effectiveness. As known in the art, other catalyst materials are subject to similar concerns at other sintering temperatures.